In an exemplary electron microcopy system having a direct detection sensor, the sensor runs at a constant speed such as 400 frames or sensor readouts per second (fps). Each sensor readout is processed by dedicated hardware in the camera to determine the location of each electron striking the detector. For this to work, in each of these sensor readouts the count rate in any area must remain below approximately one electron per twenty pixels or miscounting occurs. Read-out noise is eliminated by the electron-counting process. The system is designed such that incident electrons create a spatially-localized signal at a much higher level than the localized noise associated with read-out of the sensor. By applying a threshold to the signal, counting events above the threshold and negating noise below the threshold the read noise is eliminated. Under the above conditions, noise due to errors in counting such as false positives or missed counts is minimal. When used in an imaging mode this noise minimization, together with low spatial spread of the signal orthogonal to the incident beam, yields an excellent detective quantum efficiency. On a conventional direct detection camera system, the 400 fps data rate is far higher than a typical computer could process. One way to solve this problem is for the processing hardware to sum a number (e.g. 40) of counted frames together before sending the data to the host computer, thus yielding an effective summed-frame rate of 10 fps or an effective exposure time of 0.1 s per recorded summed-frame.
In this example for an electron microscope used in imaging mode, the minimum exposure is 0.1 seconds, the maximum summed-frame rate to the computer is 10 fps and the maximum dose rate is 20 electrons per second per pixel.
For electron-energy-loss spectroscopy (EELS) the above limitations are problematic. In a conventional, non-counting, scintillator-based detector, the spectrum is spread out in the non-spectral direction (also called the non-dispersive direction) by a few hundred pixel rows. This value is determined by balancing the competing needs of a high readout speed and low noise, requiring a smaller readout area, and the need for dynamic range and detector lifetime, requiring a larger readout area. Spectroscopy experiments often require high dynamic range to simultaneously detect regions of the spectrum that differ widely in intensity. Incident currents in the pico-Amp range on localized detector regions are common. This corresponds to many millions of electrons per second per pixel, which is much higher than the dose rate capacity of approximately 20 electrons per second per pixel shown above for the direct-detection detector when imaging in counting mode.
Spreading the spectrum out to less than the full height of the detector decreases the sensor read-out time since fewer pixels are read but, for a direct detection device in counting mode, the narrow spectrum reduces the dynamic range by the same amount that it increases the speed so in most cases there is no advantage in limiting the spectrum to smaller areas. Reducing the area does, however, reduce the lifetime of the detector so the optimal use of the detector is when the full detector is used, that is, when the spectrum spread out in both directions as in FIG. 1. Such counting-mode operation performs EELS in the absence of read-out noise and, therefore, at very high sensitivity. The invention described herein is designed to address the difficulties described above that can limit the summed frame rate and dose rate for spectroscopy.
This application is related to and incorporates by reference PCT application serial number PCT/US2015/037712 titled Electron Energy Loss Spectrometer filed by applicant Gatan, Inc.